DC local grid for wind farm

ABSTRACT

A wind farm is described in which two or more windmills are provided with each having a generator to generate electrical power. The energy supplied by the generators is output to a general a.c. grid which is set at a standard voltage. The electrical power supplied by the windmills is injected into the a.c. grid via a d.c. interconnecting grid to which a plurality of windmills are linked and with a converter. The d.c. voltage in the d.c. interconnecting grid is set as a function of the electrical power to be generated by a generator.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for generating electricpower by means of a windmill in which a generator that is equipped witha rectifier feeds direct current to a common d.c. interconnecting grid.More specifically this invention relates to a windmill system in whichtwo or more windmills supply d.c. power to a common d.c. interconnectinggrid which in turn delivers the d.c. power to a converter thattransforms the electrical power to alternating current that is thenoutput onto an a.c. grid.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,225,712 describes a method for supplying electricalpower from one or more generators driven by a windmill to an a.c. grid.The a.c. power from the generators is converted to d.c. power and thenapplied to the a.c. grid through inverters. There is no common d.c.interconnecting grid described in the '712 patent and onto which thed.c. power from the respective generators is supplied.

In the '712 patent the level of the d.c. voltages developed from thegenerators is constant and independent of the speed of rotation of thegenerator or the wind speed. The efficiency of the generator at thevarious speeds of rotation is affected, inter alia, by the voltageoutput by the generator, which means that for a single setting of thelevel of the direct current the efficiency of the generator isunnecessarily low either at a high speed of rotation and wind speed orat a low speed of rotation and wind speed, unless the generator isfitted with special and expensive provisions to improve efficiency.

SUMMARY OF THE INVENTION

With a method and apparatus of the invention the d.c. voltage of a d.c.interconnecting grid is adjusted according to circumstances whereby thegenerator of the windmill is able to operate at a higher efficiency overits entire working range and can be of simpler design. Hence, with theability to vary the voltage level of the d.c. interconnecting grid to,for example tailor it to the power being supplied by the generator,control over the various windmills used in a wind farm can be greatlysimplified. This arises because a reduction in the d.c. voltage of theinterconnecting grid enables the generator to also supply a lowervoltage and deliver its power with a lower speed of rotation of thewindmill and with a higher efficiency.

The adjustability of the voltage of the d.c. interconnecting gridenables the generator to deliver electrical power on the basis of windspeed and thus maximizes efficiency at all wind speeds. With the voltageof the interconnecting grid set on the basis of wind speeds encounteredby the windmills and the speeds of rotation of the respectivegenerators. This is particularly advantageous when the windmills arespaced far apart and/or local condition vary rapidly.

Cost savings can be obtained with a method and system of this inventionby drawing electrical power for driving local equipment at therespective windmills from the same or common electrical cable of thed.c. interconnecting grid. This electrical power can then be transformedinto a.c. when such power is required by local equipment.

Special components and techniques can be advantageously provided tointermittently store energy to enable one to safely stop a windmill whenthis is needed. Use of a common d.c. interconnecting rid enables aconvenient source of electrical power from the a.c. output grid with theuse of a power converter to startup the windmills in the farm, forexample during a lull in the wind.

The invention is explained hereinafter with reference to a particularembodiment with the aid of an embodiment shown in the drawings.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts a schematic top view of a wind farm; and

FIG. 2 is a schematic depiction of various components used in theinvention.

DETAILED DESCRIPTION OF DRAWINGS

Corresponding components have been assigned, as far as possible, thesame reference numeral.

FIG. 1 shows the top view of a wind farm comprising eight windmills 1,each provided with vanes 2 whose rotary shaft points towards theprevailing wind direction W. The windmills are set in water S and areconnected to one another and to a control building 4 via a d.c.interconnecting grid 3. The control building 4 is located on dry land L.From the control building 4 a cable runs to an interconnection point 5and thence to a public A.C. grid (not shown in any detail). Situated ontop of the control building 4 is a wind direction- and windspeed-measuring instrument 6.

FIG. 2 schematically depicts the various components. Incorporated in theschematically depicted windmill 1 an a.c. generator 8, which may beprovided with a step-up transformer 8 a and a rectifier 9. The generator8 is driven by a rotary shaft 7 to which the vanes 2 are attached (butare not shown in any detail). Also fitted can be a revolution counterfor measuring the speed of rotation of the rotary shaft 7 and thus ofthe generator 8.

In addition to the components shown here, the windmill 1 inter alia alsoincorporates a capstan drive of the vanes 2, as well as a controller forthe generator 8, if this is required for the particular generator type.The windmill 1 is further provided with means for adjusting the pitchangle of the vanes 2, in order to effect, where necessary, a lowerefficiency of the vanes 2 at high wind speeds, thus avoiding anexcessive speed of rotation of the windmill 1.

The generator 8 can be a field-controlled a.c. generator or an a.c.generator equipped with permanent magnets. If field control is used, aseparate controller is present for this purpose in the windmill 1. Thewindmill 1 further includes known means for signalling that the windmillI in question is in use, said signal being relayed to a controller 10 avia a link 11. In another embodiment, the generator 8 can be designed asa d.c. generator including the provisions necessary for this purpose. Inthat case, a separate rectifier 9 is unnecessary although there are thenadditional provisions to prevent the undesirable situation where thevanes 2 would be driven by the generator 8.

Positioned in the control building 4 is converter 10 for converting theenergy supplied to the d.c. interconnecting grid 3 into constant-voltagealternating current which, via the interconnecting point 5, is fed intothe public a.c. grid. The converter 10 is connected to the d.c.interconnecting grid 3, to which the rectifiers 9 of the variouswindmills 1 are linked. The length of the cables of the d.c.interconnecting grid 3 can, without any difficulties, be from a fewkilometres to tens of kilometres, so that the windmills 1 can besituated at as favourable a location as possible in the water S, and thecontrol building 4 can be on land L. The length of the cables in thed.c. interconnecting grid 3 does not affect the potential to transportenergy, in contrast to the situation with an a.c. interconnecting grid.

The controller 10 a is fitted on the converter 10 to control theconverter 10. By means of the controller 10 a, the converter 10 isdriven in such a way that the voltage level in the d.c. interconnectinggrid 3 is maintained at an adjustable constant value. The controller 10a is connected to the windmills via the link 11, and to one or more winddirection- and wind speed-measuring instruments 6 which are located nearthe windmills 1. The wind direction- and wind speed-measuringinstruments 6 are connected to the controller 10 a via a cable 12 alongwhich the wind data are carried. Optionally, these instruments may alsobe located on a few, for example three, windmills 1.

In the controller 10 a it is established what the voltage level in thed.c. interconnecting grid 3 must be. This voltage level may depend onthe wind speed which has been measured by means of the anemometers 6.The voltage level may also depend on the rotary shaft 7 speed ofrotation which, as measured by the revolution counter, gives anindication of the wind speed around a windmill 1 in question.Preferably, the measured values of wind speed and/or speed of rotationare averaged for the various measurements for the windmills 1 as linkedto the d.c. interconnecting grid. As a result of a particular d.c.voltage prevailing in the d.c. interconnecting grid 3, all thegenerators 8 linked via a d.c. interconnecting grid 3 produce the samed.c. voltage and therefore operate in the same operating range of thegenerator 8. Since the windmills 1 are located in each others vicinityand will encounter more or less the same wind speed, they will all beable to operate at their most efficient operating point, which allowsthe highest efficiency to be achieved in a simple manner at any windspeed.

In those situations where the wind speed is high the rotary shafts 7 ofthe windmills 1 will rotate at the maximum permissible speed of rotationof the vanes 2. This speed is determined, inter alia, by the aerodynamicrequirements and permissible load for which the vanes 2 have beendesigned. At the maximum speed of rotation, the vanes 2 will alsoprovide maximum power, and the generator 8 will likewise have to be ableto output its maximum power. This is what determines the highest levelof the d.c. voltage in the d.c. interconnecting grid 3. If the windrises further, the speed of rotation of the vanes 2 must not orvirtually not increase, and the power to be output by the generator 8will only be capable of being increased by an increase in the currentintensity. The d.c. voltage in the d.c. interconnecting grid will notincrease. In that situation, the vanes 2, if the wind rises further,will have to work at lower efficiency, which is achieved by the known(but now shown) vane adjustment. The generator 8 is of such design thatits efficiency is highest at maximum power and speed of rotation.

To ensure that in situations with less wind the efficiency of thegenerator 8 is nevertheless as high as possible, it is important tolower the voltage at which the generator 8 has to operate, which iseffected by reducing the voltage level of the d.c. interconnecting grid3. This is done in the converter 10 which is controlled for this purposeby the controller boa on the basis of the information from theanemometers 6 or the revolution counter. To achieve effective control ofthis voltage it may be of interest, moreover, to know how many on-streamwindmills 1 are connected to the d.c. interconnecting grid 3. Via a link11, each windmill 1 is connected to the controller 10 a, for example toindicate whether the windmill 1 is supplying current and, for example,to indicate the speed of rotation of the rotary shaft 7.

The link 11 is shown in FIG. 2 as a separate link used for transmittinginformation regarding the operation of the windmills 1 to the controlbuilding 4. The link 11 can also be used for transmitting control datato the windmills 1, such as wind direction data which are calculated onthe basis of the data from the wind direction measuring instruments 6.This information is used to point the windmills 1 in the wind directionW, avoiding an arrangement where each windmill 1 is fitted withfailure-prone wind direction measuring instruments. The cable fortransmitting the information can be designed as an optical cable, therebeing a facility in the windmills 1 which stops the windmill when thelink is broken.

According to another embodiment, the link 11 can be implemented as asignal present on the d.c. interconnecting grid 3, the d.c.interconnecting grid 3 acting as a network and each windmill 1extracting from the network the signal intended for said windmill.According to one embodiment, separate facilities may be provided toensure that the windmills 1 are always positioned correctly with respectto the wind direction W, i.e. when they are operational they arepointing in the wind direction W and when they are turned off they areat right angles to the wind direction W, so that the vanes 2 capture aslittle wind as possible. To this end, each mill is provided with thecapstan drive (not shown), which is supplied, for example, from the d.c.interconnecting grid 3, possibly via a converter, and which is drivenvia the link 11.

The above-described embodiment can be implemented using windmills 1 eachhaving a maximum capacity of, for example, 0.5-1.0 megawatts. The d.c.interconnecting grid 3 will have a voltage, at that capacity, of 25,000volts d.c. In the mill, the a.c. generator 8 then generates, via thestep-up transformer 8 a, a three-phase a.c. voltage of about 20,000volts, the rotary shaft 7 of the generator turning at 18-30 revolutionsper minute.

As well as in accordance with the above-described specific embodiment,the various components can be of some other, known design. As well asthe previously described a.c. generator 8, for example, a d.c. generatorcan be used instead, which is likewise provided with a controller bymeans of which the behaviour of the generator is tailored to the voltagelevel in the d.c. interconnecting grid 3.

In the previously described specific embodiment it was described how theenergy generated by the generator 8, which energy may, for example,amount to many hundreds of kilowatts, is transported to theinterconnection point 5. Present in the windmill 1 is not onlyenergy-generating equipment such as the generator 8, but alsoenergy-consuming equipment such as a drive for adjusting the vanes 2,and a capstan drive for turning the rotary shaft 7 of the vanes 2 intothe wind direction W. These drives together do not consume more than,for example, from a few up to tens of kilowatts. The controller of thewindmill 1 and the equipment present in it likewise require energy. Toprovide this energy, the windmill 1 includes an a.c. grid to which thevarious drives and equipment can be linked.

This a.c. grid can be fed in various ways. During normal operation ofthe windmill 1, the a.c. grid can be fed from one or more windings inthe generator 8, energy at the same time being fed to one or morestorage batteries for the situation where the generator 8 is at astandstill. In this arrangement, the storage batteries are sufficientlylarge for it to be possible to use them, after the generator 8 has cometo a standstill, to turn the rotary shaft 7 of the vanes 2 by means ofthe capstan drive in the direction of the wind, so that the mill willstart to turn again. Optionally, there is also enough energy for settingthe vanes at the correct pitch angle and for emergency lighting whilework is being carried out on the windmill 1. Instead of electricalstorage batteries it is also possible to use hydraulic accumulators inconjunction with a hydraulic drive.

A second way of feeding the a.c. grid in the windmill 1 is to position aconverter between the d.c. interconnecting grid 3 and the a.c. grid, bymeans of which the d.c. voltage is converted into a.c. voltage. In suchan arrangement, the said converter must be suitable for converting ad.c. voltage of many thousands of volts into three-phase a.c. voltagesof, for example, 380 volts at an output of about ten kilowatts. Thusa.c. voltage will always be present in the windmill 1 as long as thereis a voltage in the d.c. interconnecting grid 3.

A third way of feeding the a.c. grid into the windmill 1 is to use anengine-generator unit which is switched on if required.

A fourth way is to connect the a.c. grid to the interconnection point 5via cables and possibly transformers.

The above describes each windmill 1 having its own a.c. grid.Alternatively, however, the a.c. grid of two or more windmills 1 can beconnected to one another to form a local a.c. grid which can optionallybe connected, in a previously described manner, to the interconnectionpoint 5. This local a.c. grid can, if required, be provided with energyin any of the ways described above for the a.c. grid in the windmill 1.

What is claimed is:
 1. A method for generating electric power by meansof a plurality of windmills (1) each being provided with a generator (8)and a rectifier (9) for feeding direct current to a common d.c.interconnecting grid (3), the electrical d.c. power from the windmills1) being conducted via the common d.c. interconnecting grid (3) to aconverter (10) and in the converter (10) being converted intoalternating current and output to an a.c. grid (5) characterized by thestep of setting the common d.c. voltage in the d.c. interconnecting grid(3) from the respective rectifiers at a common level selected togenerate the electrical power from the plurality of windmills with anoptimum efficiency.
 2. Method according to claim 1, characterized inthat a wind speed signal indicative of wind speed at the windmills isproduced and wherein the d.c. voltage for the common d.c.interconnecting grid is set on the basis of the wind speed signal. 3.Method according to claim 1, characterized in that the wind speed signalis generated from a sensing of the speed of rotation of at least one ofthe windmills.
 4. Apparatus for generating electrical power, comprising:two or more windmills (1) each windmill being provided with a generator(8), including a rectifier (9) for supplying direct current on an outputline; a converter to convert d.c. power to a.c. power; a common d.c.interconnecting grid (3) connected to output lines of respectiverectifiers for conducting d.c. electrical power generated by therespective windmills to said converter; said converter supplying a.c.power for an a.c. grid (5); and means (10, 10 a), remotely located fromsaid windmills, for setting the d.c. voltage in the d.c. interconnectinggrid (3) for respective rectifiers.
 5. Apparatus according to claim 4,characterized in that sensors for detecting wind speed and producing awind speed signal indicative thereof are provided; and wherein saidmeans (10, 10 a) for setting the voltage in the common d.c.interconnecting grid is controlled by said wind speed signal. 6.Apparatus according to claim 4, characterized in that the means (10, 10a) for setting the d.c. voltage in the common interconnecting d.c. gridinclude one or more sensors (6) for detecting the speed of rotation ofeither one or a number of windmills (1).
 7. Apparatus according to claim4, characterized in that a windmill (1) is provided with means forconverting electrical power present in the common d.c. interconnectinggrid (3) into a form which can be used by equipment in the windmill (1).8. Apparatus according to claim 4, characterized in that a windmill (1)is provided with means for temporarily generating energy in a form whichcan be used by equipment in the windmill (1).
 9. Apparatus according toclaim 4, characterized in that a windmill (1) is provided with means forintermittently storing energy for the purpose of stopping the windmillsafely.
 10. Apparatus according to claim 4, characterized in thatconversion means are provided for feeding electrical energy present inthe a.c. grid (5) to the common d.c. interconnecting grid (3).